From what I understand from sources I've looked into, Chester was murdered cause of information he had obtained from Chris Cornell. Something to do with Hollywood pedophilia rings and the like. Apparently, Chris had obtained information and details on these pedophilia rings and was ready to come out about it along with Chester. With the influence that Chris Cornell and Chester Bennington have, this would have potentially exposed who exactly the scumbags in question are. So they both simply had to be silenced.